kni: fix possible mbuf leaks and speed up port release

kni: fix possible mbuf leaks and speed up port release

rx_q fifo can only be released by kernel thread. There may be
mbuf leaks in rx_q because kernel threads are randomly stopped.

When the kni is released and netdev is unregisterd, convert the
physical address mbufs in rx_q to the virtual address in free_q.
By the way, alloc_q can be processed together to speed up the
release rate in userspace.

In my test, it is improved from 300-500ms with a mempool that has
 131072 mbufs to 10ms(regardless of the specifications).

Suggested-by: Ferruh Yigit <ferruh.yigit@intel.com>
Signed-off-by: Yangchao Zhou <zhouyates@gmail.com>
Acked-by: Ferruh Yigit <ferruh.yigit@intel.com>